1. Field of the Invention
The present invention relates to a DC/DC converter and an audio device incorporating the same, and, more specifically, relates to a DC/DC converter used in a battery driven portable audio device incorporating a DC motor such as a portable compact disk (CD) player and in a battery driven HDD device incorporating a motor driving circuit in which a heavy load condition such as for driving a DC motor and a light load condition for merely driving a microcontroller including a MPU generating simply control signals and an IC circuit such as a memory are separated and an efficient DC/DC conversion is performed.
2. Background Art
A conventional portable CD player incorporates a DC motor and its driving circuit for rotating a CD at a predetermined rotating speed other than an audio circuit for reproducing sounds recorded on the CD. In these days such a portable CD player is further provided with a control circuit including a microcontroller, ROM and others which, for example, sets conditions of a variety of the incorporated circuits and operates the same upon receipt of a variety of operating signals such as from a control panel.
The control circuit including a microcontroller, ROM and others is usually operated at a power source voltage of about 3.about.5 V and consums a little electric power, however the audio circuit is usually driver at a power source voltage of about 3.5 V and consumes a comparatively large electric power. Further, the motor driving circuit usually requires a driving voltage of about 8 V and consumes a large electric power together with the DC motor.
With regard to a driving voltage of the DC motor itself, a lower voltage is, for example, from about 1.2 V to 2.4 V which can be taken care of by one or two dry batteries, however the motor driving circuit for driving the DC motor requires a higher driving voltage. The reason therefor is that the output stage of the motor driving circuit incorporated in such potable device uses an N type MOS transistor having a high driving capacity in order to reduce the battery power consumption. For reducing the power consumption it is necessary to reduce the ON resistance of the N type MOS transistor, for which reason the gate voltage of the N type MOS transistor has to be raised.
FIG. 5 is an example of such sorts of DC motor driving circuit, of which output circuit 10 is constituted by four N type MOS transistors TR1, TR2, TR3 and TR4 wherein the output stage transistors are piled up in an H shaped bridge structure. In the bridge portion a DC motor 11 is inserted which is represented in the drawing by the coil portion thereof. Further, in order to drive the output circuit 10 having such structure, a PWM motor control circuit 12 is usually provided.
The PWM motor control circuit 12 is constituted by a PWM control circuit 12c which receives a detection signal corresponding to the rotating speed from the motor 11 at a terminal 12e and produces a PWM control signal for maintaining the rotating speed of the motor 11 constant and two drive signal generating circuits 12a and 12b each of which generates a drive signal of which phase differs from the other by 180.degree.. Further, in order to drive respective transistors in the output stage, the output circuit 10 is provided with drive circuits 10a, 10b, 10c and 10d for driving the respective N type MOS transistors TR1, TR2, TR3 and TR4 upon receipt of the respective outputs from the drive signal generating circuits 12a and 12b.
Further, a portable audio device is provided with an audio signal processing circuit 15, a control circuit 20 including a microcontroller, a ROM and others and a power source circuit including DC/DC converters 13 and 14 and a battery 16 and feeding electric power to the DC motor drive circuit or the output circuit 10, the PWM motor control circuit 12 and the like.
In these circuits, for the first time a power source having an output voltage of 3.5 V is required for the audio signal processing circuit 15. Further, a power source having an output voltage of 2.4 V is required for driving the DC motor 11. Still further, a power source having an output voltage of 8 V is required for the N type MOS transistors TR1 and TR2 located at the upstream side in the output circuit 10. In order to meet these requirements, the DC/DC converter 13 fed from the battery 16 through a DC power source line 17 generates a voltage of 3.5 V and the DC/DC converter 14 fed from the battery 16 generates a voltage of 8 V.
In the motor control, when the motor 11 is required to be rotated in forward direction, for example, the transistor TR4 is turned ON by the output from the drive circuit 10d, the transistor TR2 is turned OFF by the output from the drive circuit 10c and the transistors TR1 and TR3 are controlled by the outputs from the drive circuits 10a and 10b in response to the pulse width of the pulse from the drive signal generating circuit 12a. Contrary thereto, when the motor 11 is required to be rotated in reverse direction, the transistor TR3 is turned ON by the output from the drive circuit 10b, the transistor TR1 is turned OFF by the output from the drive circuit 10a and the ON/OFF of the transistors TR2 and TR4 is controlled by the output of the drive circuits 10c and 10d in response to the pulse width of the pulse from the drive signal generating circuit 12b.
With thus constituted motor driving circuit, although the power consumption for the motor drive can be suppressed, three levels of power source voltages are required such that two of DC/DC converters are necessitated.
On one hand, this sort of portable audio devices is required to reduce the power consumption as much as possible because the devices tend to be driven for a long time with a battery. On the other hand, in view of easy handling of the devices many circuits corresponding to addition of a variety of functions including easy switching operation are required. When many kinds of circuits are formed in one chip by taking into account of the addition of these function circuits, the necessity of the above two DC/DC converters which require a large area on the IC chip is problematic.